Symptoms
- Connecting the monitor shows no picture at all
- Mac's display settings don't list the external monitor
- Monitor shows a 'no signal' message
- Connection works with one cable/port but not another
Likely causes
- CommonA faulty or loose cable connectionHDMI, DisplayPort, and USB-C cables can all develop internal faults.
- CommonUsing an incompatible or low-quality adapterMulti-port adapters and cheap adapters are a common point of failure.
- CommonThe monitor set to the wrong input sourceThis is an easy thing to overlook and simple to fix.
- PossiblemacOS display settings needing a manual detect or resetSometimes macOS needs a nudge to recognize a newly connected display.
- Less commonA port or graphics output issue on the Mac itselfThis becomes more likely if the same monitor works fine on another computer.
What to do
- Confirm the monitor is powered on and set to the correct input source (HDMI, DisplayPort, USB-C, etc.).
- Check that the cable is fully and securely connected at both ends.
- Try a different cable and, if using one, a different adapter or dock.
- Test the monitor with another computer if possible to rule out a fault with the monitor itself.
- In System Settings > Displays, hold Option and click 'Detect Displays' to force macOS to search again.
- Restart the Mac with the monitor already connected.
- Try a different port on the Mac if multiple are available.
- Reset NVRAM/PRAM on Intel Macs, which can resolve some display detection issues.
Tools and parts
- Known-good display cable and adapter
When to call a pro
If a known-good cable, adapter, and port combination still doesn't work, and the monitor works fine elsewhere, a technician can check the Mac's video output hardware.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.